This invention relates generally to the field of waveform measurement and sampling. More particularly, this invention relates to techniques and circuits for sampling waveforms in a waveform measurement instrument.
The field of digitally sampled waveform measurement recognizes several issues that affect how a signal can be reconstructed, measured, and displayed after it has been sampled and stored. Some of these issues are aliasing, glitch detection, etc. Aliasing of a signal occurs when the frequency of sampling of a signal is less than twice the frequency of the highest frequency component of the signal, as proven by Nyquist, and results in improper reconstruction and frequency domain representation of the signal from its discrete time samples. Glitches can go undetected without sufficient measures to capture the glitch information.
One side effect of the requirement to sample a signal at twice its highest frequency is that a large amount of storage can be required. Solutions exist that reduce the storage requirement by selectively storing only some of the signal information that is sampled while still preserving much of the high frequency information. For example, for a given sample interval in time, an existing solution stores only the minimum signal value, the maximum signal value, and a decimated sample value for that interval. By storing only a minimum, maximum, and decimated sample for each time interval rather than storing each sample, the existing solution effectively reduces the amount of storage needed for each time interval to effectively reconstruct the signal.
The minimum and maximum values stored in each time interval assure that glitches can be reconstructed within the time interval. A decimated sample value can be a signal sample that is randomly chosen (dither decimated) from each time interval or a sample from the same position (uniform) in each time interval, but not both. When a uniform decimator is used, aliasing can result because of the reduced number of samples that the compression algorithm stores (for signal reconstruction, behaves like sampling at a lower frequency). When a dither decimator is used, aliasing is effectively reduced, but certain mathematical functions such as a Fast Fourier Transform (FFT) cannot be performed and others become computationally expensive. A FFT cannot be performed with a dither decimated sampling algorithm because the sampling interval must be uniform for this frequency based mathematical operation to be applicable.
The present invention relates generally to techniques and circuits for digitally sampling waveforms in a waveform measurement instrument. Objects, advantages and features of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the invention.
A sampling technique for a waveform measuring instrument, consistent with certain embodiments of the present invention involves processing a series of digital signal samples through a uniform decimator to extract at least one uniform decimated sample value for each sample interval in a first series of sample intervals; processing the series of digital signal samples through a low frequency dither decimator to extract at least one low frequency dither decimated sample value for each sample interval in a second series of sample intervals; and processing the series of digital signal samples through a digital peak detector to extract a maximum sample value and a minimum sample value for each sample interval in the third series of sample intervals. In certain embodiments, the process further involves carrying out a reducing process by: processing the retrieved signal samples through a reducer uniform decimator to extract at least one uniform decimated sample value for each sample interval in a fourth series of sample intervals; processing the retrieved signal samples through a reducer low frequency dither decimator to extract at least one low frequency dither decimated sample value for each sample interval in a fifth series of sample intervals; and processing the retrieved signal samples through a reducer digital peak detector to extract a maximum sample value and a minimum sample value for each sample interval in a sixth series of sample intervals.
Many variations, equivalents and permutations of these illustrative exemplary embodiments of the invention will occur to those skilled in the art upon consideration of the description that follows. The particular examples above should not be considered to define the scope of the invention.